Porous pipe has been developed that weeps or drips water along its length. The hose wall is a composite formed of a dispersion of a major portion of prevulcanized elastomer particles dispersed in a continuous binder phase, usually a thermoplastic resin such as polyethylene. Channels or crevices are formed in the wall due to the lack of excess binder and the incompatibility between the binder and the dispersed particles. When the pipe is placed under pressure, the wall expands slightly and water traverses the wall and forms beads or drops on the outside surface which drip to form a continuous line source of water. Porous hose can also be used underground as a line source of irrigation at the root level or around foundations to remove excessive moisture from the soil. Representative patents describing the manufacture of porous pipe are U.S. Pat. Nos. 4,517,316, 4,616,055 and 4,615,642.
Porous pipe is finding increasing use as soaker hose manufactured with or without hose fittings for use in retail do-it-yourself (DIY), lawn and garden and professional landscape and agricultural applications. Porous hose used in outdoor applications is-found to weather and become brittle and crack after extended exposure to the outdoor ambient environment. It was believed that the aging was due to ozone in the air and was not caused by incident radiation. It is known that hydrocarbon resins such as polyethylene are degraded by ultraviolet radiation. The radiation can create free radicals resulting in chain scission or the creation of peroxy or other radicals that can degrade the polymer chains. However, it is also known that adding 3 to 5% by weight of carbon black pigment to transparent resins such as polyethylene stabilizes them from attack by ultraviolet radiation. Porous pipe is a composite containing at least 60% by weight of reclaimed rubber particles which themselves contain about 30% carbon black. Therefore, the composite hose already contains about 20% carbon black. The product is opaque. It was believed that this opacity and the high content of carbon black would provide all the ultraviolet absorption that the hose required.
It was discovered in copending application Ser. No. 07/930,345, that the addition of ultraviolet absorbers or stabilizers to the continuous binder phase results in substantial reduction in degradation of the physical properties of the wall of porous hose due to ultraviolet radiation. It was discovered on microscopic examination of the product that the binder resin formed a thin transparent film encapsulating the rubber particles. The prevulcanized particles firmly bind the carbon black and do not contribute any substantial carbon black to the binder phase. The carbon black is not available for absorption of ultraviolet light. The incident ultraviolet light apparently enters and degrades the transparent binder film leading to stress cracking and failure. The stress cracking is throughout the wall of the pipe. The ultraviolet light must flow through the binder film by light pipe principles and generate radicals that degrade the polyethylene or other binder polymer. The thinness of the film of transparent binder resin may act to concentrate or attenuate the effects of the incident ultraviolet radiation and accelerate the ageing-stress cracking process.
The addition of a small amount of an ultraviolet absorber or stabilizer is found to significantly increase the environmental stress crack resistance of the porous pipe. After accelerated exposure for an extended period, porous pipe formulated without a U.V. absorber cracked on bending. Porous pipe formulated to contain a U.V. absorber according to the invention could be bent without cracking. Samples of regular and U.V. stabilized porous pipe were tested for 6 months exposure to incident Arizona sunlight. The regular pipe was brittle and cracked. The U.V. stabilized pipe was still flexible and uncracked after six months exposure.
The U.V. stabilized pipe will substantially extend the service life of porous pipe exposed to ultraviolet light, especially in the southern and western United States which have many more days of clear skies and direct sunlight exposure at ground level. The U.V. stabilized pipe of the invention will also be able to be deployed in curved or bent configurations since the stress-crack resistance is substantially improved.